Serine protease inhibitors (serpins) play essential physiological roles in a wide range of biological processes. Serpins are researched limited in Taenia solium, although some are considered to participate in host immune responses. Tsserpins were identified as typical serpins due to the primary structure of characteristic features: the serpin motif, serpin signature and reaction centre loop (RCL). RCLs of four serpin genes (TsB6, Ts4848, Ts12383 and Ts570) contained the conserved sequences of inhibitory serpins, which may involve in immune regulation. TsEP45 differed greatly from the patterns of representative serpins, suggesting that TsEP45 may be non-inhibitory. The bioinformatic analyses were supposed that Tsserpins might be a potential antigen for diagnosis. The five recombinant Tsserpin proteins were expressed and identified reacting with Cysticercus cellulosae-positive serum samples. The indirect enzyme-linked immunosorbent assay (iELISAs) based on Tsserpins were developed and validated, one of the five Tsserpins, TsEP45, showed excellent diagnostic results with 93·33% sensitivity and 94·12% specificity, respectively. This performance was in perfect accordance with the results of the bioinformatic analysis. This study provided a comprehensive demonstration of sequences and structural-based analysis of Tsserpins. The iELISAs based on five Tsserpins were developed and compared. TsEP45 was the potential species-specific antigen for developing iELISA to detect porcine cysticercosis.

Serine protease inhibitors (serpin) play essential roles in many organisms. Mammalian serpins regulate the blood coagulation, fibrinolysis, inflammation and complement activation pathways. In parasitic helminths, serpins are less well characterized, but may also be involved in evasion of the host immune response. In this study, a Schistosoma japonicum serpin (SjB10), containing a 1212 bp open reading frame (ORF), was cloned, expressed and functionally characterized. Sequence analysis, comparative modelling and structural-based alignment revealed that SjB10 contains the essential structural motifs and consensus secondary structures of inhibitory serpins. Transcriptional profiling demonstrated that SjB10 is expressed in adult males, schistosomula and eggs but particularly in the cercariae, suggesting a possible role in cercarial penetration of mammalian host skin. Recombinant SjB10 (rSjB10) inhibited pancreatic elastase (PE) in a dose-dependent manner. rSjB10 was recognized strongly by experimentally infected rat sera indicating that native SjB10 is released into host tissue and induces an immune response. By immunochemistry, SjB10 localized in the S. japonicum adult foregut and extra-embryonic layer of the egg. This study provides a comprehensive demonstration of sequence and structural-based analysis of a functional S. japonicum serpin. Furthermore, our findings suggest that SjB10 may be associated with important functional roles in S. japonicum particularly in host-parasite interactions.

Serine protease inhibitors (serpins) are a superfamily of structurally conserved proteins that inhibit serine proteases and play key physiological roles in numerous biological systems such as blood coagulation, complement activation and inflammation. A number of serpins have now been identified in parasitic helminths with putative involvement in immune regulation and in parasite survival through interference with the host immune response. This review describes the serpins and smapins (small serine protease inhibitors) that have been identified in Ascaris spp., Brugia malayi, Ancylostoma caninum Onchocerca volvulus, Haemonchus contortus, Trichinella spiralis, Trichostrongylus vitrinus, Anisakis simplex, Trichuris suis, Schistosoma spp., Clonorchis sinensis, Paragonimus westermani and Echinococcus spp. and discusses their possible biological functions, including roles in host-parasite interplay and their evolutionary relationships.

The native form of inhibitory serpins (serine protease inhibitors) is not in the thermodynamically most stable state but in a metastable state, which is critical to inhibitory functions. To understand structural basis and functional roles of the native metastability of inhibitory serpins, we have been characterizing stabilizing mutations of human α1-antitrypsin, a prototype inhibitory serpin. One of the sites that has been shown to be critical in stability and inhibitory activity of α1-antitrypsin is Lys335. In the present study, detailed roles of this lysine were analyzed by assessing the effects of 13 different amino acid substitutions. Results suggest that size and architect of the side chains at the 335 site determine the metastability of α1-antitrypsin. Moreover, factors such as polarity and flexibility of the side chain at this site, in addition to the metastability, seem to be critical for the inhibitory activity. Substitutions of the lysine at equivalent positions in two other inhibitory serpins, human α1-antichymotrypsin and human antithrombin III, also increased stability and decreased inhibitory activity toward α-chymotrypsin and thrombin, respectively. These results and characteristics of lysine side chain, such as flexibility, polarity, and the energetic cost upon burial, suggest that this lysine is one of the structural designs in regulating metastability and function of inhibitory serpins in general.